1. Technical Field
The present disclosure relates to a network node for use in a mobile communication method and a signaling processing method performed in the network node.
2. Description of the Related Art
A voice call in a third generation partnership project (3GPP) mobile communication method has been provided using a 3GPP circuit switched (CS) network. In recent years, a Voice over Long Term Evolution (VoLTE) service, which is a voice call service using a 3GPP packet switched (PS) network, has been started.
However, areas where a VoLTE service is provided are limited. Accordingly, if a user moves outside the VoLTE coverage area during a voice call made over VoLTE (hereinafter referred to as a “VoLTE voice call”), the voice call needs to be moved to the legacy circuit switched domain. One of the technologies for allowing the transition is Single Radio Voice Call Continuity (SRVCC) described in 3GPP TS23.216 v12.6.0 “Single Radio Voice Call Continuity (SRVCC)”. An SRVCC handover procedure is described below with reference to FIGS. 1 and 2.
FIG. 1 illustrates part of the mobile communication network configuration of 3GPP. A mobile communication network illustrated in FIG. 1 includes evolved Universal Terrestrial Radio Access Network (e-UTRAN), a base station of e-UTRAN (e-nodeB), a PS network, a CS network, a base station subsystem of the CS network, and IP Multimedia Subsystem (IMS) described in 3GPP TS23.228 v12.0.0 “IP Multimedia Subsystem (IMS); Stage2” and 3GPP TS23.237 v12.6.0 “IP Multimedia Subsystem (IMS) Service Continuity”.
More specifically, in FIG. 1, e-UTRAN is a wireless access network capable of providing a VoLTE service. The PS network provides a VoLTE service. The VoLTE service includes the Packet Data Network Gateway (P-GW), Serving Gateway (S-GW), and Mobility Management Entity (MME). The CS network includes the Mobile Switching Center (MSC) and Media Gateway (MGW). The base station subsystem of the CS network includes the Radio Network Controller (RNC) and nodeB. The IMS performs a call control. The IMS includes the Call Session Control Function (CSCF) and Service Centralization and Continuity Application Server (SCC AS). Note that in FIGS. 1 and 2, the MSC and MGW are represented as one node (MSC/MGW 110). However, the MSC and MGW are represented as different nodes.
In FIG. 1, UE 100 and UE 102 (UE: User Equipment), which are mobile communication terminals, are initially connected to the PS network (note that the wireless access network, the base station, and the PS network of the UE 102 are not illustrated). When the UE 100 and UE 102 start a voice call, the Session Setup process is performed. That is, offer/answer of the Session Description Protocol (SDP) for IMS signaling are communicated between the UE 100 and UE 102 and, thus, the codec, for example, used in the voice call is negotiated. FIG. 5 illustrates an example of IMS signaling communication performed in the session setup. The IMS signaling is communicated via IMS nodes (CSCF and SCC AS in FIG. 1) of the network which the two UE are subscribed to (a home network) (not illustrated in FIG. 5). FIG. 6 illustrates the SDP offer/answer. In the example in FIG. 6, AMR and AMR-WB (described below) are offered, and the AMR is selected by the answer. If communication of IMS signaling is completed, a voice call made over VoLTE is started between the UE 100 and the UE 102. At that time, in this example, the handover (HO) of the call of UE 100 to the CS network occurs during an active voice call.
In FIG. 1, Path A, Path B, and Path C shown as solid lines are paths for speech data. In addition, in FIG. 1, paths 200, 202, 204, and 206 shown as dashed lines are the paths for signaling in the SRVCC handover process.
FIG. 2 is a sequence chart illustrating the operation of the SRVCC handover process. Each of the UE 100 and the UE 102 is initially connected to the PS network (e-UTRAN). The speech data is communicated between the UE 100 and the UE 102 via Path A. If the UE 100 moves outside the coverage area of the e-UTRAN, e-nodeB detects that event. Thus, e-nodeB exchanges signaling of the core network with RNC/nodeB via MME and MSC/MGW 110 (signaling 200 illustrated in FIG. 1, step 200 illustrated in FIG. 2 (hereinafter simply referred to as “ST200”)). In ST200, after a data path in the CS network is prepared between the nodeB and MSC/MGW 110 and, thereafter, the preparation is completed, an instruction for handover to UTRAN (the CS network) is sent from MME to the UE 100 via e-nodeB.
Concurrently with the process in ST200, the MSC/MGW 110 communicates the signaling of IMS (hereinafter referred to as “IMS signaling”) with the UE 102 via CSCF/SCC AS in the home network of the UE 100 (signaling 202 in FIG. 1, ST202 illustrated in FIG. 2). In this manner, an instruction to switch the destination of communication of the speech data of the UE 102 from the UE 100 to the MSC/MGW 110 is sent. Thus, the Path B is established.
After performing handover to UTRAN, the UE 100 exchanges signaling with the MSC/MGW 110 via RNC/nodeB (signaling 204 illustrated in FIG. 1, ST204 illustrated in FIG. 2). In this manner, the Path C is established.
After the Path C is established, the MSC/MGW 110 exchanges signaling with P-GW/S-GW via MME (signaling 206 illustrated in FIG. 1, ST206 illustrated in FIG. 2). In this manner, the Path A is removed.
As described above, the operation of SRVCC handover is performed.
As described above, IMS signaling in the home network is performed. Accordingly, if UE using a roaming service performs SRVCC in a visited network abroad, IMS signaling is sent to the home network although handover occurs in the visited network. Thus, a delay caused by, for example, the distance may occur. To address such an issue related to SRVCC and reduce the time required for data path switching, 3GPP TS22.813 v10.0.0 “Study of Use Cases and Requirements for Enhanced Voice Codecs for the Evolved Packet System (EPS)” describes an SRVCC method using Access Transfer Control Function (ATCF) enhancement (eSRVCC: enhanced-SRVCC). An example of the operations of eSRVCC is described below with reference to FIGS. 3 and 4.
FIG. 3 illustrates part of the configuration of a 3GPP mobile communication network that allows eSRVCC. Like FIG. 1, the mobile communication network illustrated in FIG. 3 includes e-UTRAN, e-nodeB, a PS network, a CS network, a base station subsystem of the CS network, and IMS. The IMS includes Access Transfer Control Function (ATCF) and Access Transfer GateWay (ATGW) in addition to CSCF and SCC AS. Note that in FIGS. 3 and 4, the ATCF and ATGW are represented as one node (ATCF/ATGW 320). However, the ATCF and ATGW are represented as different nodes.
In FIG. 3, each of the UE 100 and the UE 102 is initially connected to the PS network (Note that the wireless access network, the base station, and the PS network for the UE 102 are not illustrated). That is, VoLTE voice call is established between the UE 100 and the UE 102. At that time, the UE 100 performs handover (HO) to the CS network during the voice call.
In FIG. 3, Path A, Path B, and Path C shown as solid lines are paths for speech data. In addition, in FIG. 3, paths 300, 302, 304, and 306 shown as dashed lines are paths for signaling of the SRVCC handover process and IMS signaling.
FIG. 4 is a sequence chart illustrating the operation of the eSRVCC handover process. Each of the UE 100 and the UE 102 is initially connected to the PS network (e-UTRAN). In a system that can provide eSRVCC handover, the ATCF anchors IMS signaling, and the ATGW anchors speech data in the ATCF/ATGW 320. That is, when a voice call between the UE 100 and the UE 102 is initiated, IMS signaling for call initiation is relayed by the ATCF of the network which the UE is connected to (a visited network). If the ATCF determines that the anchor of the speech data in the ATGW is needed, the ATGW of the visited network of the UE is allocated as an anchor point of the speech data. In this manner, the speech data is communicated between the UE 100 and the UE 102 through Path A and Path B.
If the UE 100 moves away from the coverage area of the e-UTRAN, e-nodeB detects that event. Thus, the e-nodeB exchanges signaling with RNC/nodeB via MME and MSC/MGW 110 (signaling 300 illustrated in FIG. 3, ST300 illustrated in FIG. 4). In ST300, a data path in the CS network is prepared between the nodeB and MSC/MGW 110. After the preparation is completed, an instruction for handover to UTRAN (the CS network) is sent from MME to the UE 100 via the e-nodeB.
Concurrently with the process in ST300, the MSC/MGW 110 sends IMS signaling to the ATCF of the visited network of the UE 100. Thus, a path switching instruction is sent from the ATCF to the ATGW of the visited network of the UE 100 and, thus, the destination of communication of the speech data from the ATCF is switched from the UE 100 to the MSC/MGW 110 (signaling 302 illustrated in FIG. 3, ST302 illustrated in FIG. 4). That is, the Path C is established. In addition, when the path switching process to the ATGW is completed, the ATCF sends a notification signaling (IMS signaling) to the SCC AS (signaling 302 illustrated in FIG. 3, ST302 illustrated in FIG. 4).
After handover to UTRAN is completed, the UE 100 exchanges signaling with the MSC/MGW 110 via RNC/nodeB (signaling 304 illustrated in FIG. 3, ST304 illustrated in FIG. 4). In this manner, the Path D is established.
After the Path D is established, the MSC/MGW 110 exchanges signaling with P-GW/S-GW via MME (signaling 306 illustrated in FIG. 3, ST306 illustrated in FIG. 4). In this manner, the Path B is removed.
In the configuration of SRVCC illustrated in FIG. 1, even when, for example, the UE 100 that is subscribed to a Japanese cellular carrier roams to a mobile phone network in a country in Europe and make a phone call with UE 102 that is visited in the same country in Europe and if handover to a CS network occurs, IMS signaling between the MGW and the UE 102 is communicated via Japan. Accordingly, it takes a long time for the signaling between MGW and the UE 102. In contrast, in eSRVCC illustrated in FIG. 3, the MGW and the ATCF are generally located in the same country and are located in the vicinity of each other. In addition, the signaling is communicated only between the MGW and the ATCF, and signaling communicated with the UE 102 is not needed. As a result, the time required for data path switching can be reduced.
As described above, the operation of the eSRVCC handover is performed.
Examples of the speech codec used in a CS network include the Adaptive Multi-Rate (AMR) codec, which is Narrowband (NB) codec, and the Adaptive Multi-Rate Wideband (AMR-WB) codec, which is Wideband (WB) codec. The AMR and AMR-WB can be used in a packet switched system. Accordingly, AMR and AMR-WB can be used in the PS network (VoLTE).
In addition, a codec in Enhanced Voice Service (EVS) described in, for example, 3GPP TS22.813 v10.0.0 can be used in the PS network (VoLTE).
Note that the Narrowband (NB) codec mentioned above in the literature in the citation list is a codec used to perform coding and decoding processes on a digital audio signal sampled at 8 kHz. Also note that in general, the digital audio signal has an audio bandwidth from 300 Hz to 3.4 kHz. However, the bandwidth is not limited thereto. The bandwidth may be any bandwidth within the range from 0 to 4 kHz. In addition, the Wideband codec is a codec used to perform coding and decoding processes on a digital audio signal sampled at 16 kHz. Note that the digital audio signal has a bandwidth from 50 Hz to 7 kHz. However, the bandwidth is not limited thereto. The bandwidth may be any bandwidth within the range from 0 Hz to 8 kHz. Furthermore, the Super Wideband (SWB) codec is a codec that performs coding and decoding processes on a digital audio signal sampled at 32 kHz. In general, the digital audio signal has a bandwidth from 50 Hz to 14 kHz. However, the bandwidth is not limited thereto. The bandwidth may be any bandwidth within the range from 0 Hz to 16 kHz.